Though museum visitors are the most obvious source of vibrations, there are others, including those that occur after hours and at night including closing doors, traffic from a nearby road, and possibly even micro tremors which happen routinely around the world but which are so slight that seismometers are needed to detect them.
Some have questioned the vibration explanation, asking why the other statues in the same display case don't rotate the same way. Perhaps their bases are either flat or concave, preventing the figures from rotating. But there's another clue: Close observers may notice something else different about the one moving statue as compared to its three stationary cousins: It is much taller. This means that the cursed statue has a high center of gravity and thus is less stable than the others, if only slightly.
Let's be honest: If the mysterious action truly is caused by a supernatural phenomenon, it's pretty lame. In centuries past ancient curses used to be serious business, allegedly causing serious illnesses, accidents and even the sort of violent deaths you might see in a "Saw" or "Final Destination" movie. These days they can't do more than rotate a statue a few centimeters each day.